Document advanced user-defined function mapping

[Copilot]

The user-defined function mapping guide omitted several registration, type-mapping, nullability, and translation APIs. It also lacked a practical built-in JSON function example.

• Function registration

  • Document lambda-based HasDbFunction registration and default arguments.
  • Cover DbFunctionAttribute, schemas, built-in functions, and nullability metadata.

• Store-type and JSON mapping

  • Demonstrate mapping converted CLR types to function parameter store types.
  • Add a SQL Server JSON_VALUE example and clarify value-converter limitations.

[DbFunction(Name = "JSON_VALUE", IsBuiltIn = true, IsNullable = true)]
public static string JsonValue(Dictionary<string, string> json, string path)
    => throw new NotSupportedException();

var function = modelBuilder.HasDbFunction(() => JsonValue(default, default));
function.HasStoreType("nvarchar(4000)");
function.HasParameter("json").HasStoreType("nvarchar(max)");

• Translation behavior
  • Explain PropagatesNullability and link it to query null semantics.
  • Add cautions for constructing HasTranslation SQL expression trees.
  • Clarify that table-valued functions cannot use HasTranslation.

• Fixes Add missing content to user defined function mapping #2925

[Copilot]

Pull request overview

Expands advanced EF Core user-defined function mapping documentation and samples.

Changes:

• Adds lambda registration, schema, nullability, and translation guidance.
• Demonstrates SQL Server JSON_VALUE with converted CLR types.
• Clarifies HasTranslation limitations and nullability propagation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Program.cs	Adds the JSON function query.
Model.cs	Adds JSON entity, conversion, and function mapping.
user-defined-function-mapping.md	Documents advanced mapping APIs and behavior.

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[AndriySvyryd]

@copilot Add a ValueComparer

[Copilot]

Added a ValueComparer<Dictionary<string, string>> for JsonEntity.Metadata in 10f37e1, using order-independent equality/hashing and a deep-copy snapshot.